For the development of safe and long-lasting lithium-ion batteries we need electrolytes with excellent ionic transport properties. Argyrodite-type Li 6 PS 5 X (X: Cl, Br, I) belongs to a family of such a class of materials offering ionic conductivities, at least if Li 6 PS 5 Br and Li 6 PS 5 Cl are considered, in the mS cm À1 range at room temperature. Although already tested as ceramic electrolytes in battery cells, a comprehensive picture about the ion dynamics is still missing. While Li 6 PS 5 Br and Li 6 PS 5 Cl show an exceptionally high Li ion conductivity, that of Li 6 PS 5 I with its polarizable I anions is by some orders of magnitude lower. This astonishing effect has not been satisfactorily understood so far. Studying the ion dynamics over a broad time and length scale is expected to help shed light on this aspect. Here, we used broadband impedance spectroscopy and 7 Li NMR relaxation measurements and show that very fast local Li ion exchange processes are taking place in all three compounds. Most importantly, the diffusion-induced NMR spinlattice relaxation in Li 6 PS 5 I is almost identical to that of its relatives. Considering the substitutional disorder effects in Li 6 PS 5 X (X = Br, Cl), we conclude that in structurally ordered Li 6 PS 5 I the important inter-cage jump processes are switched off, hindering the ions from taking part in long-range ion transport. † Electronic supplementary information (ESI) available: Rietveld refinements and structural data, further NMR data. See Li 6 PS 5 I are also included. The lower part of the graph shows s DC T(1/T); the values given represent activation energies. For the sake of clarity, data of Li 6 PS 5 Br 0.75 I 0.25 (solid line, grey) have been plotted using an offset of +1 on the log scale.Li 6 PS 5 Cl 0.11(1) 0.18(2) 0.17(4) Li 6 PS 5 Br 0.06(1) 0.09(1) 0.10(4) Li 6 PS 5 I 0.23(1) 0.38(2) 0.18 (5) Paper PCCP

Argyrodite-type Li 6 PS 5 X (X = Cl, Br) compounds are considered to act as powerful ionic conductors in next-generation allsolid-state lithium batteries. In contrast to Li 6 PS 5 Br and Li 6 PS 5 Cl compounds showing ionic conductivities on the order of several mS cm −1 , the iodine compound Li 6 PS 5 I turned out to be a poor ionic conductor. This difference has been explained by anion site disorder in Li 6 PS 5 Br and Li 6 PS 5 Cl leading to facile through-going, that is, longrange ion transport. In the structurally ordered compound, Li 6 PS 5 I, long-range ion transport is, however, interrupted because the important intercage Li jump-diffusion pathway, enabling the ions to diffuse over long distances, is characterized by higher activation energy than that in the sibling compounds. Here, we introduced structural disorder in the iodide by soft mechanical treatment and took advantage of a high-energy planetary mill to prepare nanocrystalline Li 6 PS 5 I. A milling time of only 120 min turned out to be sufficient to boost ionic conductivity by 2 orders of magnitude, reaching σ total = 0.5 × 10 −3 S cm −1 . We followed this noticeable increase in ionic conductivity by broad-band conductivity spectroscopy and 7 Li nuclear magnetic relaxation. X-ray powder diffraction and high-resolution 6 Li, 31 P MAS NMR helped characterize structural changes and the extent of disorder introduced. Changes in attempt frequency, activation entropy, and charge carrier concentration seem to be responsible for this increase.

Lithium-thiophosphates have attracted great attention as they offer a rich playground to develop tailor-made solid electrolytes for clean energy storage systems. Here, we used poorly conducting Li6PS5I, which can be converted into a fast ion conductor by high-energy ball-milling to understand the fundamental guidelines that enable the Li+ ions to quickly diffuse through a polarizable but distorted matrix. In stark contrast to well-crystalline Li6PS5I (10–6 S cm–1), the ionic conductivity of its defect-rich nanostructured analog touches almost the mS cm–1 regime. Most likely, this immense enhancement originates from site disorder and polyhedral distortions introduced during mechanical treatment. We used the spin probes 7Li and 31P to monitor nuclear spin relaxation that is directly induced by Li+ translational and/or PS4 3– rotational motions. Compared to the ordered form, 7Li spin–lattice relaxation (SLR) in nano-Li6PS5I reveals an additional ultrafast process that is governed by activation energy as low as 160 meV. Presumably, this new relaxation peak, appearing at T max = 281 K, reflects extremely rapid Li hopping processes with a jump rate in the order of 109 s–1 at T max. Thus, the thiophosphate transforms from a poor electrolyte with island-like local diffusivity to a fast ion conductor with 3D cross-linked diffusion routes enabling long-range transport. On the other hand, the original 31P nuclear magnetic resonance (NMR) SLR rate peak, pointing to an effective 31P-31P spin relaxation source in ordered Li6PS5I, is either absent for the distorted form or shifts toward much higher temperatures. Assuming the 31P NMR peak as being a result of PS4 3– rotational jump processes, NMR unveils that disorder significantly slows down anion dynamics. The latter finding might also have broader implications and sheds light on the vital question how rotational dynamics are to be manipulated to effectively enhance Li+ cation transport.

Understanding the origins of fast ion transport in solids is important to develop new ionic conductors for batteries and sensors. Nature offers a rich assortment of rather inspiring structures to elucidate these origins. In particular, layer-structured materials are prone to show facile Li + transport along their inner surfaces. Here, synthetic hectorite-type Li 0.5 [Mg 2.5 Li 0.5 ]Si 4 O 10 F 2 , being a phyllosilicate, served as a model substance to investigate Li + translational ion dynamics by both broadband conductivity spectroscopy and diffusion-induced 7 Li nuclear magnetic resonance (NMR) spin–lattice relaxation experiments. It turned out that conductivity spectroscopy, electric modulus data, and NMR are indeed able to detect a rapid 2D Li + exchange process governed by an activation energy as low as 0.35 eV. At room temperature, the bulk conductivity turned out to be in the order of 0.1 mS cm –1 . Thus, the silicate represents a promising starting point for further improvements by crystal chemical engineering. To the best of our knowledge, such a high Li + ionic conductivity has not been observed for any silicate yet.